Most studies looking at how genitalia mismatch contributes to new species take the concept literally: if the bits don’t fit together like lock and key, matings will be unsuccessful. And if the mismatch between the gear of two groups is bad enough, they will form separate reproductive populations, and, eventually, species. But the idea, which was first tossed around more than 150 years ago, has been discounted as a possible source of new species. Differently sized or shaped genitalia is such a big change that it’s likely to come after many other speciation triggers, like mutations or long separations between populations divided by mountain ranges.